feat: tách luồng âm instrument theo môi trường — docker dùng FluidSynthWASM, standalone dùng native pyfluidsynth (+Carla VSTi)

- runtime: detect()/capabilities() trả environment (docker|standalone)
- vst_engine: render_soundfont_midi_to_audio (pyfluidsynth native, event-stream, release tail)
- plugins: POST /soundfont-render — render MIDI notes -> WAV (auth, fallback default SF)
- frontend: SonicRuntime.environment + dataset.environment; app.jsx route preview/transport:
  standalone+SF -> soundfont-render WAV (playNativeSfNote/scheduleNativeSfItem),
  docker giữ WASM, VSTi standalone giữ Carla bridge

app/static/js/services/runtime.js

@@ -20,9 +20,15 @@ window.SonicRuntime = window.SonicRuntime || { loaded: false, capabilities: null
                 var c = data && data.success ? data : { features: {} };
                 window.SonicRuntime.capabilities = c;
                 window.SonicRuntime.loaded = true;
+                // environment: "docker" | "standalone" — quyết định luồng âm
+                // instrument: docker → FluidSynthWASM (client), standalone →
+                // native (backend pyfluidsynth / Carla). Không phải docker =
+                // standalone (Windows/Linux/macOS chạy trực tiếp trên OS).
+                window.SonicRuntime.environment = c.environment || (c.docker ? 'docker' : 'standalone');
                 var html = document.documentElement;
                 html.dataset.runtime = c.runtime || 'unknown';
                 html.dataset.platform = c.platform || '';
+                html.dataset.environment = window.SonicRuntime.environment;
                 html.dataset.carla = (c.features && c.features.carla_local) ? '1' : '0';
                 if (c.features && c.features.carla_local === false) {
                     document.querySelectorAll('[data-carla-only]').forEach(function (el) {